you need to set "burn" the file onto an usb stick and then select this one as "startup disk" and from there start the installaton and change the date then in terminal once installer started. good luck with yosemite.
EDIT: i meant you need to clone the file you downloaded onto the usb stick. to do this please do as follows:
open the file with disk utility > from there click on the left pane on the yosemite file and then go to restore tab to restore it to a 8 gb usb stick.
if restore goes well, you have a bootable usb stick that should show up as a startup drive in the system settings . select the usb as startup drive and restart the computer. then the installation should proceed properly, if you set the date then onwards as suggested.